This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This collaboration has been a broadly based examination of tyrosine sulfation, a widespread posttranslational modification of proteins and peptide hormones that pass through the Golgi system of animal cells. When the project began it was limited to a number of the peptide hormones involved in the digestive process and some, for example Cholecystokinin (CCK), active both in the gastrointestinal tract and in the central nervous system. The two tissues express different but closely related receptors. The tyrosine sulfation posttranslational modification has been estimated to occur to about one percent of tyrosine residues in Rat proteins. Our initial papers focused on finding and testing rules that could be used to accurately predict tyrosine sulfation sites (1,2,3). These initial studies led to the conclusions that tyrosylprotein sulfotransferase, the enzyme that catalyzes the sulfation of tyrosine residues in proteins and peptides, has a relatively low substrate specificity and is likely to modify any tyrosine residue that is sufficiently exposed and is near negatively charged side chains. Recently it has been discovered that tyrosine sulfation of specific cytokine receptors molecules is an essential requirement for some modes of infection of the HIV-1 virus and increases the efficiency of other modes of infection (5). Additionally, tyrosine sulfation has been shown to be required for the activity of some subfamilies within the glycoprotein hormone receptors family and seems to play a similar role in other subfamilies (6). Thus we have been exploring the possibility that other receptor families may require tyrosine sulfation for either effective or full activity. We have been correlating our predictions of tyrosine sulfation binding sites with additional biochemical information about the location of receptor binding site within the protein chain. We predict that 49 tyrosines of 32 seven-transmembrane peptide receptors are sulfated. Although we did not incorporate characteristics of confirmed sulfation sites such as clustering and conservation across species into our profile (Position Specific Scoring Matrix, PSSM), our predicted sites nevertheless exhibited these characteristics. The observed conservation suggests that there are strong evolutionary pressures to preserve selected biological activity of seven-transmembrane peptide receptors. The predicted tyrosine sulfation sites predominantly occur in the extracellular tail and extracellular loop 2, regions consistent with their association with binding pockets of the receptor (4). Post-translational modification of proteins by tyrosine sulfation enhances the affinity of extracellular ligand-receptor interactions important in the immune